ZRT is an acronym which stands for Zero Radius Turn. When used in conjunction with a vehicle, ZRT means a vehicle having a very tight or sharp minimum turning radius. Vehicles such as riding mowers often desirably have ZRT capability due to the frequent and sharp turns needed to mow around trees, bushes and other obstructions. Accordingly, there have been many ZRT riding mowers.
Most ZRT riding mowers have a frame including at least one drive wheel on each side of the frame. The drive wheels on each side of the frame are independently powered so that one can be operated in a forward direction while the other is stopped or even operated in reverse. This is what provides ZRT capability. Operating one drive wheel on one side of the frame forwardly while simultaneously stopping or operating the drive wheel on the other side of the frame rearwardly causes the mower to spin about its center in a very sharp turn.
Many ZRT mowers have what is known as a twin stick control system. Two control sticks are provided side-by-side with each stick controlling one of the drive wheels. When the sticks are advanced together forwardly out of their neutral position, both drive wheels are operated forwardly to cause the mower to move forwardly. Steering is accomplished by advancing one stick more than the other. A ZRT type turn can be achieved by advancing one stick forwardly without advancing the other stick or even pulling back sharply on the other stick.
A cruise control lever is provided on some ZRT mowers for establishing a cruise control speed. The twin sticks of the control system are spring biased forwardly out of their neutral position and into contact with the cruise control lever. Thus, the operator of the mower can take his hands off the sticks and they will remain biased up against the cruise control lever to cause the vehicle to drive forwardly at whatever speed is determined by the position of the cruise control lever. To slow down or stop, the operator has to grab the control sticks and move them back away from the cruise control lever against the biasing force of the springs.
U.S. Pat. No. 3,459,273 to Leinhauser and U.S. Pat. No. 4,589,249 to Walker show twin stick control systems on ZRT vehicles with a cruise control lever against which the control sticks are biased.
One disadvantage with the above noted cruise control lever is that the mower cannot be temporarily accelerated out of its cruise speed without changing the position of the cruise control lever. Inherently, this loses the current cruise control setting, requiring that the operator try to reset the cruise control lever after the temporary acceleration is finished. The need to first move the cruise control lever to temporarily speed up and to then reset, or attempt to reset, the cruise control lever to its original position is cumbersome.
Moreover, having the control sticks biased up against a cruise control lever complicates the slight feathering motions of the sticks which are required to slightly adjust the direction of the mower. It is often the case that one control stick has to be momentarily feathered ahead of the other stick to steer the vehicle, but this cannot be done when both sticks abut against the cruise control lever. The second stick could be slightly and momentarily retarded instead of feathering the first stick in advance of the other stick. However, retarding the stick in this manner, against the bias of the spring forcing the stick forward, is not an obvious action, and, in fact, is the reverse of what most unskilled operators would expect to do. Accordingly, the known twin stick systems of this type having such a cruise control lever are difficult to use, particularly for an unskilled operator.
Some ZRT mowers have twin control sticks that are not biased forwardly against any type of cruise control lever. Some of these sticks are not biased at all, but simply remain at whatever position they are put in by the operator. Effectively, a cruise control speed can be set simply by advancing the control sticks to a desired position and leaving them there.
However, such a system has the same disadvantage as the earlier described system of not being able to temporarily accelerate the mower without losing the current setting of the sticks. The sticks can always be advanced to temporarily speed up, but doing so removes them from their initial position. The operator then has to try and reset them in that position to resume the original speed. However, there is nothing to guide the operator in where to reset the sticks and so regaining the original speed is difficult to do without hunting back and forth with the sticks until the speed seems right.
Another problem with this latter system is that there is no upper limit on the cruise control speed that can be set. The sticks could be pushed to their far forward position and left there. There is no way to set a cruise control speed that the operator cannot easily change.